add testcase for issue #1646

core/src/test/java/com/alibaba/fastjson2/issues_1600/Issue1646.java

@@ -0,0 +1,29 @@
+package com.alibaba.fastjson2.issues_1600;
+
+import com.alibaba.fastjson2.JSONObject;
+import com.alibaba.fastjson2.JSONReader;
+import org.junit.jupiter.api.Test;
+
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue1646 {
+    @Test
+    public void test() {
+        String s = "{\"test\":{null:\"a\",\"b\":\"c\"}}";
+        Bean bean = (Bean) JSONObject.parseObject(s, Bean.class, JSONReader.Feature.AllowUnQuotedFieldNames);
+        assertEquals("a", bean.getTest().get("null"));
+    }
+
+    public class Bean {
+        private Map<String, String> test;
+        public Map<String, String> getTest() {
+            return test;
+        }
+
+        public void setTest(Map<String, String> test) {
+            this.test = test;
+        }
+    }
+}